having an issue but i will clarify as to the vehicle in question fist. 97 BMW 318i 4cyl, 140,589 miles auto. Items replaced recently include the thermostat and the water pump less then 500 miles ago. So I'm having some issues with the radiator over flow tank. i ran the car for 30 minutes then when I arrive at my destination (home) the water starts to come up out of the overflow tank quickly. The vehicle during those 30 minutes of driving didn't show any change in temp, stayed steady at half way. What would cause the water to want to come up so quickly if the car never showed signs of heating up, especially with a new pump and thermostat? There is no oil loss or any signs of oil mixing with water at all. Have I been running too much water and the vapor is an issue?
When water comes out of the overflow tank, my first reaction is that you have a head gasket leak. Not all such leaks mix oil and coolant. One failure mode is a one-way leak (due to pressure) leaking exhaust into the coolant; if this happens, the gasses in your cooling system have nowhere to go but out the overflow tank (which is not tightly sealed, unlike the rest of the system). I would recommend a combustion leak testing system like the following to determine if this is your problem:
It's also possible you have a much simpler problem like a bad radiator cap with a problem in the return pressure valve.